Spider Tension Turns Caustic

January 28, 2012

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Photographer: Rick Stankiewicz
Summary Author: Rick Stankiewicz

January 2012 Earth Science Picture of the Day Viewer's Choice This fishing spider (Dolomedes tenebrosus) was captured along the surface of a northern Ontario lake while I was on my own fishing expedition in June of 2006. Note how the approximately one-half-inch (13 mm) spider uses surface tension to walk on water. Two if its eight legs have actually broken through the surface of the water. The dimpling of the water surface appears out of focus, but it's actually the concave meniscus of the surface making it appear that way. The pink granite beneath the water made a nice background to photograph both this unique predator and the refraction of sunlight below it. What appears to be a shadowy reflection is caused by the meniscus at the surface bending the light and focusing it away from the area at those points under the spider. The chromatic edges (bright outline) of these dark areas delineate the caustic surface.

Dolomedes spiders are covered with unwettable (hydrophobic), short, velvety hairs. This allows them to use surface tension to stand or run on the water, like pond skaters. When these spiders move beneath the water, air becomes trapped in the body hairs. This forms a thin film over the whole surface of the body and legs, giving them the appearance of fine polished silver. Like other spiders, Dolomedes breathe with both lungs, which are found beneath their abdomens. Their lungs open into the air film, allowing the spiders to breathe while submerged. The trapped air makes them very buoyant. So if they let go of a rock or a plant stem they're holding on to, they float toward the surface where they pop onto the surface, completely dry. It's quite an amazing adaptation for an aquatic existence.

Photo details: Camera Maker: NIKON; Camera Model: E995; Focal Length: 31.0mm; Aperture: f/5.8; Exposure Time: 0.0043 s (1/234); ISO equiv: 100; Exposure Bias: none; Metering Mode: Matrix; Exposure: program (Auto); Flash Fired: No; Orientation: Normal; Color Space: sRGB; Software: E995v1.